<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>7-尺寸位置操作</title>

    <style>
        body {
            height: 2000px;
        }

        .back {
            position: fixed;
            width: 50px;
            height: 50px;
            background-color: pink;
            right: 30px;
            bottom: 100px;
            display: none;
        }

        .container {
            width: 900px;
            height: 500px;
            background-color: skyblue;
            margin: 400px auto;
        }
    </style>
</head>

<body>

    <div class="back">返回顶部</div>
    <div class="container"></div>

    <script src="../3-jq效果/js/jquery.min.js"></script>
    <script>
        $(function () {

            $(window).scroll(function () {
                let a = $('.container').offset().top;
                let a1 = $(document).scrollTop();
                console.log(a1);
                if (a1 >= a) {
                    $(".back").fadeIn();
                    console.log('a', 1123);
                } else {
                    $('.back').fadeOut();
                }
            });

            $('.back').click(function () {
                $('body,html').stop().animate({
                    scrollTop: 0
                });
            })

        });
    </script>
</body>

</html>